What is the diagnostic significance of an elevated sp/AP (summating potential to action potential) ratio in electrocochleography (ECochG) for Meniere's disease?

Diagnostic SP/AP Ratio in Ménière's Disease Using Electrocochleography

Primary Recommendation

ECochG should not be routinely ordered to establish the diagnosis of Ménière's disease, as the test lacks sufficient sensitivity and specificity, with results that fluctuate throughout the disease course. 1, 2

Understanding the SP/AP Ratio

The SP/AP ratio measures the summating potential (generated by cochlear hair cells) relative to the action potential (cochlear nerve response) during electrocochleography. 1 An elevated ratio theoretically indicates endolymphatic hydrops, the pathophysiologic hallmark of Ménière's disease.

Traditional Diagnostic Thresholds

• Amplitude ratio >0.4 is conventionally considered abnormal 3, 4
• Area curve ratio >1.94 has been proposed as an alternative measurement 4

Clinical Performance and Limitations

Sensitivity Issues

The diagnostic sensitivity of ECochG is disappointingly low across all disease stages:

• Definite Ménière's disease: Only 57-67% show abnormal SP/AP amplitude ratios 3, 5
• Probable Ménière's disease: Only 40-53% demonstrate elevated ratios 3, 5
• Critical implication: Approximately 30-33% of patients with definite Ménière's disease have normal ECochG results 3

Area Ratio vs. Amplitude Ratio Controversy

While some studies suggested the SP/AP area curve ratio might improve sensitivity 6, 7, 4, the most recent evidence using transtympanic ECochG (the gold standard technique) contradicts this:

• Area ratio sensitivity: 27.7-43.9% across disease stages 5
• Amplitude ratio sensitivity: 39.6-57.1% across disease stages 5
• The amplitude ratio actually outperformed the area ratio in all patient groups 5

When ECochG May Be Considered

According to the American Academy of Otolaryngology-Head and Neck Surgery, ECochG has limited utility but may provide complementary information in specific scenarios: 1

Appropriate Use Cases

• Atypical presentations where clinical diagnosis remains uncertain after standard evaluation 1, 2
• Lateralization uncertainty when determining which ear is affected, particularly before considering ablative interventions 1, 8
• Pre-ablative assessment as part of comprehensive vestibular function evaluation before destructive procedures 1

Tests That Should Be Used Instead

The mandatory diagnostic approach includes:

• Audiometry with pure tone thresholds and word recognition scores - this is essential and non-negotiable 1, 2
• MRI of internal auditory canals to exclude vestibular schwannoma and other mimics 2
• Clinical diagnostic criteria: Two or more spontaneous vertigo episodes (20 minutes to 12 hours), documented fluctuating low-to-mid frequency sensorineural hearing loss, and fluctuating aural symptoms 2

Critical Clinical Pitfalls

Common Errors to Avoid

• Over-reliance on ECochG: Normal results do not rule out Ménière's disease given the 30-40% false-negative rate 3, 5
• Timing issues: Results fluctuate throughout the disease course, making interpretation unreliable 2
• Cost-benefit imbalance: The specialized equipment, training requirements, and limited diagnostic yield make routine use unjustifiable 1
• Delayed diagnosis: Waiting for ECochG results when clinical criteria are already met delays appropriate treatment 1

When Results Won't Change Management

ECochG should be avoided when:

• Clinical diagnostic criteria for definite Ménière's disease are already met 1, 2
• The test result will not alter treatment decisions 1
• Vestibular migraine or other diagnoses are more likely and should be addressed first 2

Evidence Quality Assessment

The 2020 American Academy of Otolaryngology-Head and Neck Surgery guideline represents the highest quality evidence, explicitly recommending against routine ECochG use. 1, 2 This guideline supersedes older research studies that suggested potential utility of area curve ratios, particularly given that the most recent research (2009) using optimal transtympanic technique showed area ratios performed worse than amplitude ratios. 5

The clinical diagnosis of Ménière's disease remains fundamentally based on history, audiometry, and exclusion of other pathology - not on ECochG findings. 1, 2